MSC are not immunogenic and escape recognition by alloreactive T cells and NK cells. They have low level expression of HLA class I and no expression of HLA class II. They do not express co-stimulatory molecules and are unable to induce T cell mediated immune response, and therefore can be used as universal donor cells. Allogeneic cells have been widely used in clinical trials in both immune modulation and tissue repair. The advantage of using allogeneic cells is that donor age can be controlled. The literature indicates that MSC harvested from donors older than 40 years are inferior. Since many of the patients being treated with a cranioplasty are greater than 40 years of age, the use of allogeneic cells in this study is preferable. Clinical human trials and animal studies support the use of allogeneic cells.